Structures for radiative cooling

ABSTRACT

Various aspects as described herein are directed to a radiative cooling device and method for cooling an object. As consistent with one or more embodiments, a radiative cooling device includes a solar spectrum reflecting structure configured and arranged to suppress light modes, and a thermally-emissive structure configured and arranged to facilitate thermally-generated electromagnetic emissions from the object and in mid-infrared (IR) wavelengths.

RELATED DOCUMENTS

This patent document is a divisional under 35 U.S.C. §120 of U.S. patentapplication Ser. No. 13/829,997 filed on Mar. 14, 2013 (U.S. Pat. No.9,709,349) which claims benefit under 35 U.S.C. §119 to U.S. ProvisionalPatent Application Ser. No. 61/726,777, entitled “Structures ForRadiative Cooling” and filed on Nov. 15, 2012; each of these patentdocuments is fully incorporated herein by reference.

OVERVIEW

Aspects of the present disclosure are directed toward radiant daytimecooling. In certain more specific embodiments, a structure facilitatesfar-field radiation at particular wavelengths while blocking radiationat solar wavelengths. Additionally, aspects of the present disclosureallow for cooling of buildings and similar structures.

Aspects of the present disclosure utilize radiative cooling techniquesthat exploit the natural transparency window for electromagnetic wavesin the Earth's atmosphere to transport heat from terrestrial objects.These techniques can be used to facilitate passively cooling even attemperatures that are well below the ambient air temperature. Particularaspects are premised upon the recognition that the blackbody spectralradiation wavelengths for common terrestrial temperatures (0-50° C.) areat or near wavelengths where the atmosphere is nearly transparent.

For buildings (and other structures), cooling is a larger issue when thetemperature is higher and when the building is exposed to directsunlight, both of which happen during daytime. Daytime radiative coolingcan therefore be significantly more useful than nighttime cooling, butis also often much more challenging due to the problem of absorbed solarradiation.

FIGURES

Various example embodiments may be more completely understood inconsideration of the following detailed description in connection withthe accompanying drawings.

FIG. 1 depicts a radiative cooling device for cooling an object,consistent with embodiments of the present disclosure;

FIG. 2 depicts an example of methods and structures for coolingterrestrial structures such as buildings, automobiles and electronicdevices where heat management is an issue, consistent with embodimentsof the present disclosure;

FIG. 3 depicts atmospheric transmission at normal incidence vs.wavelength and normalized blackbody spectral radiance of a 0° C. and a50° C. blackbody emitter, consistent with embodiments of the presentdisclosure;

FIG. 4 depicts emissivity of an experimental design for radiative coolershown at normal incidence, consistent with embodiments of the presentdisclosure;

FIG. 5 shows an alternative configuration for providing cooling forvarious structures, consistent with embodiments of the presentdisclosure; and

FIG. 6 depicts a cooling system using a heat that cools a buildingstructure and/or internal devices, consistent with embodiments of thepresent disclosure.

While the disclosure is amenable to various modifications andalternative forms, specifics thereof have been shown by way of examplein the drawings and will be described in detail. It should beunderstood, however, that the intention is not to limit the disclosureto the particular embodiments described. On the contrary, the intentionis to cover all modifications, equivalents, and alternatives fallingwithin the scope of the disclosure including aspects defined in theclaims.

DESCRIPTION OF EXAMPLE EMBODIMENTS

Various aspects of the present disclosure are directed towardsapparatus, methods of use, and methods of manufacturing of radiativecooling structures.

Certain aspects of the present disclosure are directed towardsmethods/apparatuses that include a radiative cooling device for coolingan object. A solar spectrum reflecting structure is configured andarranged to suppress light modes within the structure from coupling tosources that are externally located relative to the object being cooled.The particular light modes that are suppressed can be targeted toprohibiting coupling of incoming solar radiation by including at leastsome wavelengths in the visible, near IR, and ultraviolet spectrum(solar spectrum). A thermally-emissive structure is configured andarranged to facilitate thermally-generated emissions from the object andin mid-infrared (IR) wavelengths. At least a portion of both the solarspectrum reflecting structure and the thermally-emissive structure areintegrated into a constitution to both prohibit the coupling of theincoming solar spectrum to the object and facilitate the emission fromthe object and in mid-IR wavelengths. This type of integration can beparticularly useful for a number of different reasons, some of which arediscussed in more detail herein.

Radiative cooling can include nighttime cooling, however, such coolingoften has a relatively limited practical relevance. For instance,nighttime radiative cooling is often of limited value because nighttimehas lower ambient temperatures than daytime, and therefore, there isless of a need for cooling. Accordingly, aspects of the presentdisclosure are directed toward macroscopically planar photonicstructures that selectively enhance mid-IR emission of light,specifically in the atmospheric transparency window, and also suppressabsorption of light in the wavelength range of 300 nm-4 μm, i.e., thesolar spectral range. Such structures can be useful for a variety ofapplications including, but not limited to, passively coolingterrestrial structures such as buildings, homes and electronics in thedaytime and the nighttime.

In particular embodiments, the structure is macroscopically planar innature and includes layering and texturing at the nanometer tomicrometer scale. For instance, the structure can include materialswhose properties are given by a frequency-dependent dielectric constantand are configured to enable sub-wavelength interference and near-fieldlight coupling between constituent layers so as to form spectral regionswith a suppressed number of light modes. This suppression can be in theform of photonic band gaps that lead to reduced absorption of solarlight. The structure can also include materials, whose properties aregiven by a frequency-dependent dielectric constant and are configured toenable sub-wavelength interference and near-field light coupling betweenconstituent layers so as to form spectral regions with an enhancednumber of light modes. These enhanced light modes can be used toincrease the emission of light in the 8-13 μm wavelength range. Thereare a number of configurations and mechanism for achieving thesuppression or enhancement of light modes. A few, non-limiting examplesare discussed hereafter.

To enhance the emissivity in the 8-13 μm wavelength range or in thewavelength range supported by a blackbody with temperatures in the rangeof 250−350° K, a first solution uses a grating or a photonic crystal, tocouple surface phonon-polaritons to free-space light modes. This leadsto the enhanced emission of light in the 8-13 μm or in the wavelengthrange supported by a blackbody with temperatures in the range of250-350° K. The enhanced emission of light is embodied in the emissivityspectrum.

According to another solution, a finite multilayer stack is used thatincludes two or more materials. The stack is configured to exploit anear-field coupling of light mode, and sub-wavelength interference. Thisallows for the enhancement of the emission of light in the 8-13 μmwavelength range or in the wavelength range supported by a blackbodywith temperatures in the range of 250-350° K. The enhanced emission oflight is embodied in the emissivity spectrum.

To suppress absorption in the 300 nm-4 μm wavelength range, one solutionuses a multilayer stack consisting of two or more materials, to exploitnear-field coupling of light modes, and sub-wavelength interference, tosuppress absorption of solar light (300 nm-4 μm). The suppressedabsorption of light is embodied in the emissivity/absorption spectrum.

Turning now to the figures, FIG. 1 depicts a radiative cooling devicefor cooling an object, consistent with embodiments of the presentdisclosure. The cooling device structure is shown by a side view 100 andalso as a top-down view 150. In particular, the metallic (Ag) portion ofthe structure shown in the side view 100 can be placed in thermalcontact with the object being cooled (the contact at least thermallycontacting, but also can be physically contacting). The opposite/topportion of the structure (Quartz) can be exposed to sunlight and also tothe atmosphere and paths for radiating thermal energy.

An example and experimental embodiment of the structure of FIG. 1includes (from top to bottom): A quartz grating layer with thickness of2.5 μm; a SiC grating layer with thickness of 8 μm. Both of these layerscan have a periodicity of 6 μm and can have square air rectangles of 5.4μm width. The top-down view 150 shows a two-dimensional gratingstructure in this layer, which can be configured and arranged to enhanceemissivity that is useful for radiative cooling. For instance, theenhanced emissitivity can be within the range of 8-13 μm where theatmosphere is substantially transparent. This type of grating has beenfound to be useful for strong emissivity over a broad range of angles ofincidence (e.g., 0-80°).

Immediately below the grating layers is a multilayer stack: 3 sets of 5bilayers (15 layers in total) of varying thicknesses. The first set of 5bilayers has thicknesses of 25 nm of TiO₂ and 35 nm of MgF₂; the secondset 50 nm of TiO₂ and 70 nm of MgF₂; the third set of 75 nm of TiO₂ and105 nm MgF₂. The multilayer stack is designed to suppress the absorptionof solar light throughout the solar spectrum. For instance, these layerscan create photonic band gaps that prevent solar light from propagatingthrough the structure.

The particular materials, patterning and thicknesses can be varied andstill provide the ability to enhance or suppress the relevant lightmodes in a single integrated constitution as shown in FIG. 1. The use ofa single integrated constitution can be particularly useful for avoidingproblems stemming from a solution that might use multiple differentcomponents separated by significant physical distances and notintegrated into a single constitution. For instance, a reflectivecovering foil placed over a radiative structure can complicate the totalcooling system to the point of severely limiting its versatility ofapplication and durability (e.g., a covering foil might range inthickness from several microns to a fraction of a millimeter, and acompromise between IR transmission and solar reflection may lead toundesirable consequences).

FIG. 2 depicts an example of methods and structures for coolingterrestrial structures such as buildings, automobiles and electronicdevices where heat management is an issue, consistent with embodimentsof the present disclosure. As depicted, various embodiments can providea passive way of cooling such structures, which can be useful fordramatic energy savings. For instance, experimental testing supportsthat the performance by the daytime radiative cooler can be at least:P_(cooling)(T_(ambient))=50 W=m² at T_(ambient)=300° K.

In comparison, solar panels that operate at 20% efficiency can generateless than 200 W/m² at peak capacity. In certain conditions, the passivedaytime radiative coolers proposed here could be thought of as solarpanel substitutes (or supplements) that reduce the demand on a rooftopsolar system by reducing the need for air conditioning (cooling)systems.

FIG. 3 depicts atmospheric transmission at normal incidence vs.wavelength and normalized blackbody spectral radiance of a 0° C. and a50° C. blackbody emitter, consistent with embodiments of the presentdisclosure. FIG. 4 depicts emissivity of an experimental design forradiative cooler shown at normal incidence, consistent with embodimentsof the present disclosure. FIGS. 3 and 4 suggest that by placing aradiative cooler on rooftops, the buildings could receive significantcooling. As a non-limiting estimate of the building-level energy impactof such a radiative cooler, the effect of a passively cooling rooftop inthe daytime on the building's air conditioning needs can be modeled asfollows: 1) a peak cooling load of approximately 6 kW (e.g., in Chicagoand Orlando) for canonical 2233 ft² one-story homes and 2) the radiativecooler is operating at its peak cooling rate. For 40 m² of daytimeradiative cooler on the rooftop (20% of a total of 200 m² availablerooftop space), 32% of the house's air conditioning needs can be offsetduring the hottest hours of the day.

Reducing the air conditioning load at peak hours can be particularlyuseful for reducing the grid's overall need for dirty ‘peak-power’sources that kick in to cover extra power needs in the summer. Moreover,such radiative cooling structures can reduce overall energy demands fromcommercial buildings such as factories, warehouses and data centers,lending a significant hand to the nation's energy efficiency goals. Airconditioning alone is believed to represent 23% of the power usage ofresidential and commercial buildings, or 16.33% of the total electricpower usage of the United States as of 2011. A 10% reduction in airconditioning needs system wide via thorough implementation of daytimeradiative cooling structures would thus represent a 1.6% reduction inthe total electric power usage of the country, or 61.7 TWh. This wouldbe equivalent to reducing the need for 7 GW of power generating capacityoverall.

FIG. 5 shows an alternative configuration for providing cooling forvarious structures, consistent with embodiments of the presentdisclosure. To take advantage of the integrated constitution andassociated advantages, the cooling structure can be placed within (oron) a variety of different support structures. One such structure isshown in FIG. 5. This type of support structure can allow the coolingstructure to be located above the surface of the object being cooled.This can be particularly useful for situations where the surface of theobject is used for other purposes. Consistent with various embodiments,the support structure can be configured for portability so as to allowfor simple installation and removal.

Moreover, the support structure could include adjustable elements (e.g.,a rotational support portion) that allow the cooling structure to beoptimally oriented. In some instances, the orientation could be adjustedfor different times of day or even different times of the year. Forexample, the cooling structure can be uninstalled or oriented to reducecooling when the ambient temperature is below a threshold value, as mayoccur during certain times of the year or simply during a cold front.Other possibilities include the use of such structures for cooling oftemporary structures (e.g., temporary buildings for large events) or useon mobile structures while motionless and removed during motion (e.g.,to avoid damage due to wind shear or objects that might strike thecooling structure during movement).

Accordingly, automobiles represent another area where cooling energycosts can be reduced with a daytime radiative cooler. Although allvehicles could potentially benefit, electric vehicle (EV) battery rangecould benefit greatly from a reduction of air conditioning needs. It isbelieved that air-conditioning can reduce an EV's charge depletion rangeby up to 35%. Experimental modeling suggests that an air conditioningload of 1000 W for small cars could then be reduced 10% by covering 2 m²of the car's surface.

Another potential application is for extra-terrestrial cooling. In outerspace, radiation is the dominant mechanism of heat exchange andtemperature regulation. A device operating in space (e.g., orbitingsatellite, spaceship or landing probe) which produces heat has thepotential to benefit from the use of a radiative cooling structure thatwould allow it to cool more efficiently and/or obtain a pre-specifiedequilibrium temperature.

FIG. 6 depicts a cooling system using a heat exchanger that cools abuilding structure and/or internal devices, consistent with embodimentsof the present disclosure. As shown, the building can be cooled using aheat exchange system that conducts/convects heat away from the internalsof the building to the roof. For instance, liquid (e.g., water) can becycled through the system and used to cool the building and/or internalcomponents, such as racks of servers. When the liquid reaches theradiative structure it cools through passive radiation as discussedherein. A heat pump (or similar device) can be used to further increasethe cooling capabilities of the system.

Embodiments of the present disclosure are directed toward these andother mechanisms for passively cooling structures even in extremely hotenvironments. This can be useful for cost and energy savings over thelifetime of buildings and other structures or objects.

Consistent with experimental examples discussed herein, assuming anradiative cooler is operating at its peak cooling rate, then 40 m² ofdaytime radiative cooler on the rooftop (20% of a total of 200 m²available rooftop space), one can offset 32% of a house's airconditioning needs during the hottest hours of the day.

The embodiments and specific applications discussed herein may beimplemented in connection with one or more of the above-describedaspects, embodiments and implementations, as well as with those shown inthe appended figures.

For further details regarding cooling efficiency and energy costs,reference is made to the below-listed documents, which are fullyincorporated herein by reference.

A. Burdick, “Strategy guideline: Accurate heating and cooling loadcalculations,” Tech. Rep., U.S. Dept. of Energy: Energy Efficiency andRenewable Energy, 2011.

U. E. I. Administration, “Annual Energy Outlook 2012,” Tech. Rep., U.S.Dept. of Energy, 2012.

L. R. J. R. Robb A. Barnitt, Aaron D. Brooker and K. A. Smith, “Analysisof off-board powered thermal preconditioning in electric drivevehicles,” Tech. Rep., National Renewable Energy Laboratory, 2010.

R. Farrington and J. Rugh, “Impact of vehicle air-conditioning on fueleconomy, tailpipe emissions, and electric vehicle range,” Tech. Rep.,National Renewable Energy Laboratory, 2000.

T. M. Nilsson and G. A. Niklasson, “Radiative cooling during the day:simulations and experiments on pigmented polyethylene cover foils,”Solar Energy Materials and Solar Cells, Vol. 37, No. 1, pp. 93-118,1995.

S. Catalanotti, V. Cuomo, G. Piro, D. Ruggi, V. Silvestrini, and G.Troise, “The radiative cooling of selective surfaces,” Solar Energy,Vol. 17, No. 2, pp. 83-89, 1975.

Various embodiments described above, and shown in the figures may beimplemented together and/or in other manners. One or more of the itemsdepicted in the present disclosure can also be implemented in a moreseparated or integrated manner, or removed and/or rendered as inoperablein certain cases, as is useful in accordance with particularapplications. In view of the description herein, those skilled in theart will recognize that many changes may be made thereto withoutdeparting from the spirit and scope of the present disclosure.

What is claimed is:
 1. A radiative cooling device for cooling an object comprising: a solar spectrum reflecting structure configured and arranged to suppress light modes, thereby inhibiting coupling of incoming electromagnetic radiation, of at least some wavelengths in the solar spectrum, to the object; a thermally-emissive structure configured and arranged to facilitate thermally-generated electromagnetic emissions from the object and in mid-infrared (IR) wavelengths; and at least a portion of each of the solar spectrum reflecting structure and the thermally-emissive structure being integrated as a constitution configured and arranged to both prohibit coupling of the incoming electromagnetic radiation to the object and facilitate the thermally-generated electromagnetic emissions from the object and in mid-IR wavelengths.
 2. The device of claim 1, wherein the solar spectrum reflecting structure includes a multi-layer structure of alternating materials configured and arranged to suppress absorption of at least some wavelengths in the solar spectrum by the device, including radiation in wavelengths in at least one of visible spectrum, near IR, and ultraviolet spectrum.
 3. The device of claim 1, wherein the radiative cooling device is configured and arranged to provide radiative cooling exceeding 20 W/m² at an ambient temperature of 300° K with an equivalent of direct sunlight striking the device.
 4. The device of claim 1, wherein the suppressed light modes inhibit coupling of incoming electromagnetic radiation of wavelengths from 300 nm to 4 μm.
 5. The device of claim 1, wherein the thermally-emissive structure is configured to facilitate thermally-generated emissions by coupling surface phonon-polaritons to free-space light modes.
 6. The device of claim 1, wherein the thermally-emissive structure is configured and arranged to facilitate thermally-generated emissions by exploiting near field coupling of light modes and sub-wavelength or wavelength-scale interference.
 7. The device of claim 1, wherein the constitution is less than 50 mm thick.
 8. The device of claim 2, wherein the multi-layer structure of alternating materials is configured and arranged to suppress the absorption in at least some wavelengths in the visible spectrum.
 9. A method for cooling an object comprising: suppressing light modes using a solar spectrum reflecting structure of alternating materials each having different dielectric constants that prohibit coupling of incoming electromagnetic radiation, of at least some wavelengths in the visible, near-IR and ultraviolet spectrum, to the object; and facilitating thermally-generated emissions that originate from the object, that are in mid-IR wavelengths and are generated above the solar reflecting structure of alternating materials.
 10. The method of claim 9, further including a step of providing radiative cooling of at least 20 W/m² at an ambient temperature of 300° K and with an equivalent of direct sunlight striking the structure of alternating materials.
 11. A method for cooling an object, the method comprising: using a solar spectrum reflecting structure to suppress light modes and thereby prohibit coupling of incoming electromagnetic radiation, of at least some wavelengths in the solar spectrum, to the object; using a thermally-emissive structure to facilitate thermally-generated electromagnetic emissions from the object and in mid-infrared (IR) wavelengths; and wherein at least a portion of each of the solar spectrum reflecting structure and the thermally-emissive structure are integrated as a constitution to both prohibit the coupling of the incoming electromagnetic radiation to the object and facilitate the thermally-generated electromagnetic emissions from the object and in mid-IR wavelengths.
 12. The method of claim 11, wherein using the solar spectrum reflecting structure further includes suppressing absorption of at least some wavelengths in the solar spectrum, including radiation in wavelengths in at least one of visible spectrum, near IR, and ultraviolet spectrum, by using a multi-layer structure of alternating materials.
 13. The method of claim 11, further including a step of providing radiative cooling exceeding 20 W/m² at an ambient temperature of 300° K with an equivalent of direct sunlight striking the solar spectrum reflecting structure.
 14. The method of claim 11, wherein using the solar spectrum reflecting structure to suppress light modes further includes inhibiting coupling of incoming electromagnetic radiation of wavelengths from 300 nm to 4 μm.
 15. The method of claim 11, wherein using the thermally-emissive structure further includes facilitating thermally-generated emissions by coupling surface phonon-polaritons to free-space light modes.
 16. The method of claim 11, wherein using the thermally-emissive structure further includes facilitating thermally-generated emissions by exploiting near field coupling of light modes and sub-wavelength or wavelength-scale interference. 